Federal scientists studying the Great Salt Lake have found some of the highest levels of mercury ever measured anywhere -- prompting concern about some of the migratory birds that feed on the lake's brine shrimp. U.S. Geological Survey and Fish and Wildlife Service researchers were initially gathering information on selenium in the lake, but decided also to test the samples for mercury. Concentrations of methylmercury -- the element's most poisonous form -- exceeded 25 nanograms per liter of water. Fish consumption warnings have been issued when there was just 1 nanogram per liter.